The paper reviews detection methods in optical fiber systems, including noncoherent, differentially coherent, coherent, and hybrid detection, with a focus on coherent detection and its advantages in achieving high spectral and power efficiency. Coherent detection, particularly with polarization multiplexing, maximizes spectral and power efficiency by encoding information in both amplitude and phase, or in the in-phase (I) and quadrature (Q) components of a carrier. The paper discusses the benefits of coherent detection, such as the ability to fully recover the received signal field using dual-polarization homodyne or heterodyne downconversion, and the use of digital signal processing (DSP) for compensation of transmission impairments like chromatic dispersion and polarization-mode dispersion (PMD). DSP-based receivers offer adaptability to time-varying impairments and support advanced forward error correction codes. The paper also compares modulation formats, highlighting the advantages of polarization-multiplexed quadrature-amplitude modulation (QAM) in terms of spectral and power efficiency. Experimental results demonstrate the feasibility of coherent detection at high bit rates, and the paper concludes by discussing the practical implementation complexities of single- and multi-carrier systems.The paper reviews detection methods in optical fiber systems, including noncoherent, differentially coherent, coherent, and hybrid detection, with a focus on coherent detection and its advantages in achieving high spectral and power efficiency. Coherent detection, particularly with polarization multiplexing, maximizes spectral and power efficiency by encoding information in both amplitude and phase, or in the in-phase (I) and quadrature (Q) components of a carrier. The paper discusses the benefits of coherent detection, such as the ability to fully recover the received signal field using dual-polarization homodyne or heterodyne downconversion, and the use of digital signal processing (DSP) for compensation of transmission impairments like chromatic dispersion and polarization-mode dispersion (PMD). DSP-based receivers offer adaptability to time-varying impairments and support advanced forward error correction codes. The paper also compares modulation formats, highlighting the advantages of polarization-multiplexed quadrature-amplitude modulation (QAM) in terms of spectral and power efficiency. Experimental results demonstrate the feasibility of coherent detection at high bit rates, and the paper concludes by discussing the practical implementation complexities of single- and multi-carrier systems.